The distracted, quick-draw reality of how people use iPhone apps means that sessions get chopped up into quick sprints, wedged between other activities. When a friend suggests going to the roller derby on Saturday, you break from conversation to dash the rendezvous into your calendar, then quickly return to chit-chat. When the wait at the post office gives you a spare minute, you scan your email, Twitter account, and favorite website before it's your turn at the counter. Get in, get out.
The best apps fold neatly into the fabric of a busy schedule. This demands a special degree of efficiency in the interface—get me there in just a tap or two—but it also demands visual simplicity. In the context of scattered attention and a distracting environment, you can't expect people to have the time or patience to study the screen.
As with all things, there are exceptions. Some will spend hours at a time losing themselves in an immersive game. Others will spend long stretches engrossed in an ebook novel or tapping out thoughtful notes. But those very same apps—game, ebook reader, notebook—will just as likely be used for a 30-second sprint in the same person's next session. This means that even apps that encourage longer, more contemplative interactions should anticipate and design for quick hits. (You'll explore more about the specific mindsets that people bring to mobile apps starting in Mobile Mindsets.)